Ammonia (NH3), which has high content of hydrogen atoms per unit of volume, has been used occasionally in the past as a fuel for internal combustion engines and fuel cells. Ammonia appears to be an excellent hydrogen source and can play a crucial role in the hydrogen economy of the future.
Ammonia (NH3) is about three times less expensive than hydrogen per volume of stored energy, and, similarly to hydrogen, it can be combusted in an environmentally benign way, exhausting only water and nitrogen. Moreover, the energy content of ammonia per unit of volume is comparable to that of gasoline which makes it a fuel attractive for transportation applications.
The idea of an ammonia-based economy in which fossil fuels are converted to ammonia and then used as a clean (synthetic) fuel in transportation and remote applications is important, due to the fact that ammonia production and consumption become CO2 free.
Regarding the ammonia-fuel storage on-board of vehicles, in addition to the known storage in a pressurized liquid phase, ammonia can be absorbed in porous media like metal ammine complexes, e.g., hexaamminemagnesium chloride, Mg(NH3)6Cl2. The hexaamminemagnesium chloride is formed simply by passing ammonia over anhydrous magnesium chloride at room temperature and the absorption and desorption of ammonia is completely reversible. The ammine can be shaped in the desired form and can store 9% hydrogen per weight (9 kg H2 in 100 kg) or 100 kgH2/m3. Another option is to combine ammonia with borane and form ammonia borane (NH3BH3) which is in the form of a powder under normal atmospheric conditions; by dissolving it in water ammonia borane emanates hydrogen and ammonia compounds.
Ammonia can fuel directly alkaline and solid oxide fuel-cells (SOFC) to produce steam and some NOx as exhausts; the NOx can be reduced by known methods. Moreover direct ammonia solid electrolyte fuel-cells were recently developed. For the ammonia use on-board of the PEM fuel-cell vehicles, the most cited option is the ammonia catalytic decomposition into nitrogen and hydrogen, reaction that is thermally driven by a 350° C. heat source. This heat can be obtained by catalytic combustion of a small part of the generated hydrogen. As an alternative option, hydrogen can also be obtained via ammonia electrolysis using a part of the electrical energy produced by the fuel cell.
Internal combustion engines (ICE) fuelled directly with ammonia must have special features because the ammonia's flame speed is slow. Some recent studies have demonstrated that homogeneous charge compression ignition (HCCI) technology may provide 40-50% efficiency for a large range of compression ratios, i.e., 40:1-100:1.
However, if decomposed even partially, ammonia can fuel regular internal combustion engines because the mixture of hydrogen, nitrogen, ammonia, and air has comparable combustion characteristics as gasoline. Furthermore, possibilities exist to separate the hydrogen from nitrogen after decomposition and thus to feed the cylinder with almost pure hydrogen; the combustion process is improved and NOx emission minimized in this way.
There are a series of patents regarding the use of ammonia as a fuel. An example is shown in U.S. Pat. No. 7,157,166 which refers to a direct ammonia fuel-cell system to generate electricity. The fuel-cell operates at high temperature and possesses a proton conducting membrane. Therefore there is no need of an afterburner like in the common solid oxide fuel-cells, because all protons of the ammonia molecules supplied at the anode can potentially diffuse through the membrane. The remaining gas at the anode is pure nitrogen, while the cathode exhausts steam and air (with less oxygen).
There are a series of patents regarding internal combustion engines with improved combustion that operate with a mixture of gasoline and ammonia. For example, in the solution proposed by U.S. Pat. No. 4,478,177 ammonia is stored on board in a liquefied phase. Using exhaust gases ammonia is evaporated and disintegrated and thereafter injected in the intake manifold. U.S. Pat. No. 2,559,814 presents an idea to cool the intake air with an ammonia spray that absorbs heat while vaporizing; this mixture is combined thereafter with gasoline for combustion. A similar patent is presented by U.S. Pat. No. 4,223,642. Injecting ammonia into diesel fuel is cited in the literature as a method of obtaining a cleaner combustion with improved efficiency and reducing wear and costs associated with maintenance.
There are some patents referring to ammonia cracking for hydrogen and nitrogen generation on vehicles and other applications. U.S. Pat. No. 5,976,723 investigates the catalytic cracking over various metals, while U.S. Pat. No. 5,055,282 focuses only on ruthenium catalyst. The idea of decomposing ammonia and fuelling internal combustion engines with the resulting hydrogen and nitrogen has been patented in 1938 by U.S. Pat. No. 2,140,254. In 1949 an ammonia decomposition unit that uses an electrical resistance to provide the decomposition heat, as illustrated in U.S. Pat. No. 2,578,193. The unit is connected to an ammonia bottle and exhausts a mixture of mainly nitrogen and hydrogen. The device is claimed to be so safe that it can be used at home, “even by the children”, for filling balloons with light gas.
It has to be mentioned at this point that, using ammonia-fuel implies only regular and well known safety measures because it has a very narrow flammability range. If released into the atmosphere, ammonia's density is lighter than that of air and thus it dissipates rapidly. In addition, because of its characteristic smell the nose easily detects it in concentrations as low as ˜5 ppm. For instance, U.S. Pat. No. 3,979,681 regards ammonia expansion and injection into the soil for agriculture applications, where ammonia is used as fertilizer. It is worth pointing out that in such common agricultural applications, some quantities of ammonia escape into the atmosphere; the practice demonstrates that the operators can cope easily with the danger associated with ammonia's toxicity in such conditions.
Ammonia is proposed as a refrigerant for vehicle air conditioning based on ammonia-water absorption refrigeration systems. The advantage of such a system is that there is no use of a compressor that would take large amount of useful shaft work of the engine. For providing cooling, only the exhaust gas heat is recovered.
Moreover, ammonia is proposed as a reduction agent of NOx emission of internal combustion engine. To this respect ammonia is stored on-board in various forms (urea, nitrogen monoxide, metal amines).
There is an unclassified US military application of ammonia as a fuel for small portable fuel-cells used by soldiers to power electrical equipments. There, ammonia is mentioned as safe hydrogen storage with no essential risks associated with the toxicity.
In the following text, some trials regarding the development of ammonia fuelled cars are mentioned. The first ammonia car was built in 1933 by the Fiat in Italy. The engine is adapted from a regular gasoline one, where the exhaust gases are passed through a heat exchanger that heats-up ammonia to the thermal decomposition temperature, i.e., approx 350-400° C. The resulting decomposition products (nitrogen, hydrogen and traces of ammonia) are mixed with air in a carburetor and thereafter fed into the cylinder.
There is a US-based company developing and testing ammonia cars, using alkaline fuel-cells with hydrogen obtained on-board from ammonia thermal cracking. The heat generated by the fuel-cell is recovered and used for ammonia decomposition. The car is equipped with a 60 kW fuel-cell and an ammonia fuel tank of 8.7 gallons and range up to 200 miles. There were initially some ammonia trucks tested in the USA in 1933 and some more recently in 2004, using a mixture of 80% ammonia and 20% gasoline.